Fly fishing requires relatively long lengths of line to be reeled in and out, and the reels used must make it possible to reel the line out easily by pulling the line off the reel, and must enable the line to be reeled in quickly by acting on a lever or other means for rotating the spool in the appropriate direction.
Known reels for fly fishing are either of the automatic type including an electric motor or a spring for reeling in the line, in which case they are expensive, heavy, and relatively fragile, or else they are of the type that is manually driven by means of a lever of the crank handle type in which case they are relatively noisy and do not enable the line to be reeled in quickly.
In addition, manually controlled reels are often of the "vertical" type, i.e. the reel is fixed on the fishing rod in such a manner that the plane of rotation of the spool contains the axis of the fishing rod, with the axis of rotation of the spool being horizontal and perpendicular to the fishing rod. That type of reel suffers from the drawback that traction exerted on the line to reel it out tends to cause the fishing rod to rotate in the hand of the fisher and causes the line to change direction quite considerably (which line is fragile), thereby often causing the fisher to bend the wrist to accompany traction motion of the line.
There also exist "horizontal" type reels in which the axis of rotation of the spool is substantially perpendicular relative to the axis of the rod and intersects said axis, the plane of rotation of the spool being parallel to the axis ofthe fishing rod and extending substantially horizontally when the fishing rod is held horizontally.
That type of reel can be used equally well by a person who is left handed and by a person who is right handed, and the line can be reeled out more easily. However, in known reels of that type, the actuator levers of the means for rotating the spool to reel in the line are unhandy in use, and the line is reeled in relatively slowly. They are also quite noisy.